sorry if this is very basic question but since I haven't used any group buying website myself I don't know how do they earns money? To be more clear:
Thanks for your time :)
Groupon is taking a percentage on the deal. Let say that you run a business and want to use the Groupon service. Imagine that you sell guided visit to a museum. The price is 50 $ per visitor. On Groupon the price will be 25$, and you will earn 10$ (the rest is for Groupon).
The numbers here are for the purpose of illustrating the general idea, the actual numbers can be quite different.
Revenue share is based on the individual strategy of the deal sites and Groupon tends to stay around 50/50 with others ranging from that to 80/20 (more for the merchant).